The silicon avalanche photodiode (Si APD) has a fast time response and high sensitivity in the near infrared region. APDs are available with active areas from 0.2 mm to 5.0 mm in diameter and low dark currents (selectable). Photo courtesy of Hamamatsu

Tunable Optical Filters
Liquid Crystal Filters:
An electrically controlled liquid crystal elements to select a specific visible wavelength of light for transmission through the filter at the exclusion of all others.

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AO Tunable Filters:
The AOTF range of acoustooptic (AO) devices are solid state optical filters. The wavelength of the diffracted light is selected according to the frequency of the RF drive signal.

Confocal Microscopy

sec. switching time
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Monochromators
Mirrors
Czerny-Turner design

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1. Slit Width (mm) is the dimension of the slits. Exit Slit 2. Bandpass is the FWHM at the selected wavelength. 3. The dispersion is the factor to convert slit width to bandpass.

Raman Scatter of Water
Vibrational modes of water
Resonant Stokes Raman scattering

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Energy for the OH stretch vibrational mode in water (expressed in inverse wavenumbers): 3200 cm-1

Simple formula to calculate the wavelength of the Raman peak:
(1) (2) Insert the excitation wavelength (eg. 490 nm) in the following equation: The result specifies the position of the Raman peak in nanometers (i.e. the Raman peak of water is located at 581 nm for this excitation wavelength of 490 nm.

Two UV selected calcite prisms are assembled with an intervening air space. The calcite prism is birefringent and cut so that only one polarization component continues straight through the prisms. The spectral range of this polarizer is from 250 to 2300 nm. At 250 nm 39 there is approximately 50% transmittance.

Filter Choice For Steady-State as well as Time-Resolved Polarization Measurements Make sure, absolutely no scattered excitation light is detected ! Use a high quality emission filter Why ? I// - I P= I// + I

The Pockels Cell is an electro-optic device that uses the birefringent properties of calcite crystals to alter the beam path of polarized light. In applying RF power, the index of refraction is changed and the beam exiting the side emission port (0 polarized) is enhanced or attenuated. In applying 47 RF the output light becomes modulated.

Channel # (Each 50 ps wide) (1) The pulse width and instrument response times determine the time resolution. (2) The pulse frequency also influences the time window. An 80 MHz pulse frequency (Ti:Sapphire laser) would deliver a pulse every 12.5 ns and the pulses would interfere with photons arriving later than the 12.5 ns time.